'Free will offerings' in the Bible

Speak to Aaron, and to his sons, and to all the children of Israel, and say to them, Whoever he may be of the house of Israel, or of the strangers in Israel, that will offer his oblation for all his vows, and for all his free-will-offerings, which they will offer to the LORD for a burnt-offering:

Beside the sabbaths of the LORD, and beside your gifts, and beside all your vows, and beside all your free-will-offerings, which ye give to the LORD.